hull shape of cruising power v sail cat

I've read before about how a powercat has a very different hull shape to a sailing cat (ignoring tris). I can understand that truth if you are planning on flying around at 20+ knots (on either design!).

However, say you were designing for cruising (not gulping fuel); why *must* the powercat have a different shaped hull to a sailing cat if the average speed is going to be a cruisy 8-9 knots for both? (see also the other thread about average cruising speed of about 7-8.5 knots)

Note please the *must* in the previous para - I can see how the shape *can* be different, say if you're not using boards but minikeels and still want the boat to tack successfully. I can see that engine placement may effect rudder placement for both, and that they may be different. Ignore anything above water of course (eg. mast placement, beam, etc).

Another way to ask the question: if you were designing a **cruising** boat that could either be a powercat *or* a sailing cat, could the underwater hull shape be the same?


(sigh: more caveats to preempt the replies - cruising speed avg of about 7-9 knots, can mean short-time sailing speeds of 12-16 knots in some conditions)

 

How big a cat?

 

did you read Malcolm Tenant's article?
http://www.catamarans.com/news/2006/04/CatComparison.asp

 

Thanks for that - I'd read it years ago but forgot.

I think Malcolm has answered me:

(my emphasis) and he also explains a little why boat length (size) makes a difference. I was thinking of around 28-35' boat size originally, but I didn't think it mattered for the principle (I was wrong).

So I'm taking away the point that unless you want to regularly go cruising at more than 15kn, for a smallish boat of around 10m or less, a sailing cat and power cat *could* have the same underwater shape.

 

Great article.

Does anyone know where I can find clear pictures of the CS hull that Malcolm describes. The image in the article is not very good and I tried searching Google but can't find any good images of a CS hull out of the water.

Are there any articles or books published that go into more detail on the design/dimensions of a CS hull?

 

The fundamental difference between a power cat and a sailing cat is that the power cat has a known amount of propulsion power available at all times. As such, it can afford to have a hull form that has humps in the resistance curve as long as the resistance is minimized at the operating speed.

A sailing cat cannot afford to have such humps as it would then never accelerate over the hump in lower windspeeds, so it requires a form that has low wave making resistance, combined with minimum wetted surface area. These are contradictory requirements so slenderness ratio is directly a function of intended operating speed.

Of course, if you choose an operating speed that is well below the hull speed, then the optimum shape for both would converge, though the location of the driving force will dictate differences in buoyancy distribution.

Using "must" in your original statement negates all the above, since you can power and sail any shape you like, but I am assuming you are looking for some level of optimisation.

 

Most bizarre opinion, and explanation of said opinion, ive ever heard...???

I dont suppose you can clarify this a bit more can you?

 

exchanced some mail with Malcolm Tennant in 08 too were he cinicly mentiont other boats named to have good caracteristics since they used his design

not bad for a motorsailor, were i always wonder what they do using power and sail! on hullshape discussions can be endless, here an older one on the CS hull: http://www.boatdesign.net/forums/motorsailers/cs-hull-sailing-performance-35387-2.html
he and his site is gone and with it drawings, consider however that the cordova with CS hull was from 83 and since, even more optimised, has been used in many bigger cats and ferry's

 

Jipster you mean this website?
http://www.tennantdesign.co.nz/

 

yes that site, part of it etched in my mind still
might be my explorer bucking but cant even get a ping

 

What bit do you want clarification on?

Hull resistance is, simplistically, a combination of form drag (principally wave-making) and friction drag. Hence optimisation for a particular speed will involve balancing form drag vs friction drag. Typically, low speed drag is dominated by form drag, high speed by friction drag

Reducing form drag requires a long slender shape, but this has high wetted surface area.

Reducing friction drag involves reducing surface area, but this requires full forms with high form drag.

The hull speed is a function of form and length. A longer hull or a more slender hull has a higher hull speed than a shorter or fatter hull. Exceeding hull speed requires driving the hull through its own bow wave, which is associated with a large increase in drag. As speed increases further, drag will reduce relative to speed, and hence significantly higher speeds can be achieve with installed power not much above that required to overcome the bow wave. This is typical for a planing hull.

So, if you have a fixed, known amount of power, either because you have an engine, or you only intend to use the boat in a lot of wind, then you can design the hull to exceed its hull speed provided the amount of power available is sufficient.

However, for a slower sailing boat, the amount of available power is variable from zero to a maximum, and performance needs to be maintained across the entire range, hence sailing hulls tend to favour reducing form drag and eliminating humps in the resistance curve, but by doing so limit their top speed due to excessive wetted surface area.

The OP specified a speed that would probably be below his hull speed, and as such he will not be able to take advantage of a hull form that can reduce it's WSA at such speeds. In this case the optimised hull for power or sail would be ostensibly similar (slender), but in fact the sailing hull will require a forward distribution of buoyancy to resist the pitching moment generated by the rig, whilst the power hull will require exactly the opposite since the pitching moment generated by the propeller thrust line creates a pitching moment in the opposite direction. This is why sailing boats tend to squat excessively under power.

 

http://dominocatamaran.blogspot.com.au/?view=snapshot

 

I spoke to Malcolm about this before staring mine as I only wanted 10-12 knots and shallow draft (something CS does not offer)
I also flattened and squared up the last 15 % of my hulls to help prevent any squat.

 

Thanks for the great pictures of the CS hull.

On the underside of the flat rear section it looks like there is a small concave area. What purpose does that serve? Does it extend all the way forward to the canoe or just to the rudder?

I notice the square transom, is that important or can that area be rounded in either direction? I figure the sides of the rear flat area should stay sharp.

 

What are the things I circled in red?